Glass Block as Field Flattener for Head Mounted Disply

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budda
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Glass Block as Field Flattener for Head Mounted Disply

Post by budda »

Hi,

I made an interesting observation when looking into a large tropical fish tank at a local shopping centre.

The tank was rectangular and very wide.

As I looked closer to the glass I noticed the back wall developed a distinct curvature effect. I checked around the side to see if it was really curved but it wasn't.

So, of course, I thought this principle could be applied to field curvature correction in a virtual reality display, like the Oculus Rift.

I made a mock up of such a system with a glass block, a printed sheet and two close-up filter lenses.

The field correction was excellent as the viewing field came into and out of focus uniformly. The viewing field was limited to 90 degrees because of the small lens size used. The mirror reflection at the side of the block allowed me to look further afield if necessary.

I used a red filter from a pair of 3D glasses to cancel out the chromatic aberation from the lens to ensure the focus level was extra clear.

My equipment for the experiment was as follows:

1. Glass Block 50x50x80 mm (with laser etched sailing boat inside it)
2. Close up filter lens 18 Dioptre 37mm (10 + 8)

The lens vertex was held 25mm from the top of the glass block for optimum focus at infinity.

The weight could be a difficulty if applied to a head mounted display, however an acrylic block would halve the weight and make it easier to machine as required.

The advantages are many.

1. Being a flat field corrector means the associated optics or viewpoint can be arbitrarily moved around without fuss.

2. The level of field correction can be adjusted by varying the plate thickness or stack.

3. The introduced optical aberations from the glass block appear negligible.

4. It can still be combined with a plano-concave field flattener in a hybrid arrangement if necessary.

5. It may be retrofittable to the Oculus Rift.


Thanks.
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brantlew
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Re: Glass Block as Field Flattener for Head Mounted Disply

Post by brantlew »

interesting, but it does sound on the heavy side
geekmaster
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Re: Glass Block as Field Flattener for Head Mounted Disply

Post by geekmaster »

There are a number of active patents covering what you described. Selling such a device may get the attention of patent trolls who would love to extort a large cash settlement from you.
budda
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Re: Glass Block as Field Flattener for Head Mounted Disply

Post by budda »

Hi,

Another attractive feature of the 'glass block' as a field flattener, is one I did not mention.

It is possible to use the block as 'anti-chromatic aberation' - or, as a chromatic aberation corrector.

When light comes out from a higher refractive medium, as in the case of a 'glass block', the blue-end of the spectrum appears further outward than the red-end of the spectrum. Think of it as an anti-rainbow effect.

I use the acronym 'breadbox' to remember this - as opposed to 'rainbow'.

Breadbox - blue to red looking inward to the centre.

Rainbow - red to blue looking inward to the centre.

Acrylic has a low dispersion which makes it difficult to achieve a large chromatic correction effect.

However, the acrylic can be replaced by polycarbonate, which has a significant dispersion.

The advantage of using dispersive optical elements is that the necessary chromatic correction applied to the screen image is minimised and the resulting image through the lens should appear better.

Another way to tailor the net dispersion for a given total block thickness, is to combine two or more thicknesses of dispersive materials within the block. For instance, an acrylic slab can be placed next to a polycarbonate slab to form the block which achieves this same effect.

In this way the net chromatic correction can be better engineered for the desired application - in head mounted displays.

Thanks.
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Fredz
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Re: Glass Block as Field Flattener for Head Mounted Disply

Post by Fredz »

I don't see how this is working, can you post photos or a short video of the effect ?
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equusvenustas
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Re: Glass Block as Field Flattener for Head Mounted Disply

Post by equusvenustas »

I wonder if its possible to use some kind of aerogel as field flattener, that way you should able to solve the problem of weight but most aerogels aren't transparent or not totally transparent and are very expensive :|
budda
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Re: Glass Block as Field Flattener for Head Mounted Disply

Post by budda »

I wonder if its possible to use some kind of aerogel as field flattener
Field curvature is a function of the lens' surface powers, being their curvatures and refractive indices.

An aerogel would be optically transparent as far as refraction is concerned.

Refraction is heavily influenced by the electron density of the material. Thats why highly refractive glasses are made of heavy metals such as lead and the rare earths.

Perhaps diffraction optics, like holographics, would be the way to go as a lightweight alternative.
budda
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Re: Glass Block as Field Flattener for Head Mounted Disply

Post by budda »

I don't see how this is working, can you post photos or a short video of the effect ?
The lateral chromatic aberation effect can be simply demonstrated by looking at a black line on white paper and viewing the line at various radial positions using a high powered magnifying lens.

The radially outward side of the black line will have a red fringe and the radially inward side of the black line will have a blue fringe. The degree of the lateral red-blue displacement increases with radial extent.

The black line can hide the blue fringe that is pulled in from the outside of the black line, and it can likewise hide the red fringe that is pulled outward from the inside of the black line too. If the black line is thin, then maybe the two internally shifted red and blue colours combine to make a purple line.

This is what I call the 'rainbow' effect, where red appears laterally shifted in the radially outward direction (like a real rainbow), and blue appears laterally shifted toward the inward or central direction of the rainbow.

To offset this lateral chromatic aberation at seen through the high powered lens in a head mounted display, the display on the screen can be be drawn so the colour shift is reversed or somewhat corrected. This is called chromatic aberation correction.

However, this is a digital correction and is limited to the pixel geometry of the screen. There is a rasterisation of the chromatic error correction.

It may be possible to provide optical chromatic aberation correction, in whole or in part, by using the polycarbonate block to smoothly correct the chromatic aberation caused by the viewing lens.

If this works .... then all or part of the digital chromatic error correction displayed on the screen could be offloaded to the polycarbonate block, and the chromatic error correction could be done off-screen.

Thats the principle.
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Re: Glass Block as Field Flattener for Head Mounted Disply

Post by Fredz »

Ok, thanks for the explanation. So basically you think that using a sheet of plexiglas in front of (or behind ?) the aspheric lens should be able to correct chromatic aberration without doing it in software ? Could be nice as a DIY project, although I'm not sure using an optical coating wouldn't be more effective for Oculus VR considering their recent funding. I guess brantlew can't comment on this unfortunately.
budda
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Re: Glass Block as Field Flattener for Head Mounted Disply

Post by budda »

So basically you think that using a sheet of plexiglas in front of (or behind ?) the aspheric lens should be able to correct chromatic aberration without doing it in software ?
1. Polycarbonate not acrylic (plexiglas).

A basic reference for optical materials and chromatic dispersion is:

http://en.wikipedia.org/wiki/Abbe_number
Abbe numbers are used to classify glass and other optically transparent materials. For example, flint glass has V < 50 and crown glass has V > 50. Typical values of V range from around 20 for very dense flint glass, around 30 for polycarbonate plastics, and up to 65 for very light crown glass, and up to 85 for fluor-crown glass. Abbe numbers are a useful measure of dispersion for visible light
The lower the Abbe number is, the higher is the chromatic dispersion.


2. The polycarbonate slab would have to be between the screen and the lens.


For practicality, there is room only between the screen and viewing lens.

The lens could be made achromatic, with chromatic aberation corrected, but it would be nearly impossible to build or wear. This is because you need to make it from two lenses with a power difference of 20 dioptres, and a single lens with 20 dioptres power is already at the extreme end of the lens range.

Achromatic lens: http://en.wikipedia.org/wiki/Achromatic_lens


Thanks.
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Re: Glass Block as Field Flattener for Head Mounted Disply

Post by Fredz »

budda wrote:The lens could be made achromatic, with chromatic aberation corrected, but it would be nearly impossible to build or wear. This is because you need to make it from two lenses with a power difference of 20 dioptres, and a single lens with 20 dioptres power is already at the extreme end of the lens range.
I know about Achromats and dispersion in optics, I meant using special coating on aspheric lenses to correct the chromatic aberration without the need of several lenses (called hybrid plastic aspheres).

General explanation here : http://www.laserfocusworld.com/articles ... --one.html

More details here : http://www.nittohkogaku.co.jp/english/z ... hybrid.php

Can be bought here : http://www.edmundoptics.co.uk/optics/op ... enses/3200
budda
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Re: Glass Block as Field Flattener for Head Mounted Disply

Post by budda »

Hi,

Thanks for the information about these advanced lenses.
I meant using special coating on aspheric lenses to correct the chromatic aberration without the need of several lenses (called hybrid plastic aspheres).
I think the basics still apply.
This is because you need to make it from two lenses with a power difference of 20 dioptres, and a single lens with 20 dioptres power is already at the extreme end of the lens range.
Its the optical equivalent of trying to put two pounds of mince in a one pound bag.

I cannot see it being practical for a larger aperture, wide-angle viewing lens for head mounted displays.


Thanks.
zalo
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Re: Glass Block as Field Flattener for Head Mounted Disply

Post by zalo »

Isn't this essentially how achromatic lenses work?
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